The last time editors at the Atlantic magazine heard from correspondent Clare Morgana Gillis, she was somewhere in eastern Libya, covering the fighting between rebels and government troops. That was Monday, April 4.

Since then, the sound at the end of her cellphone has been an ominous one: silence.

Gillis, a freelancer who also works for USA Today, is among five Western journalists who have been missing and remain unaccounted for. Although the news is scant and sketchy, three of them, including Gillis, have been spotted by Western sources in a government detention camp in Tripoli.

The whereabouts of two others, South African news photographer Anton Lazarus Hammerl and American freelance journalist Matthew VanDyke, a Baltimore native who has been missing since mid-March, are unknown.

“We’re really concerned,” Atlantic editor James Bennet said on Thursday. “We’re hoping they’re released soon, but there hasn’t been an update for two days.”

The apparent detention of the journalists highlights the continuing hazards of covering the ongoing turmoil in the Middle East and northern Africa. The Committee to Protect Journalists, an organization that tracks government harassment of the news media, said there have been more than 500 attacks on reporters since unrest began to spread in late December. Nine journalists have been killed in the region since then, the organization said, including Zakariya Rashid Hassan al-Ashiri, a blogger in Bahrain who died while in government custody this week.

Among others, four New York Times journalists, including former Washington Post reporter Anthony Shadid, were detained by forces loyal to the Libyan government in March. They were beaten by soldiers, held for six days and eventually freed from custody with the intervention of Turkish diplomats. During the uprising in Egypt, CBS News correspondent Lara Logan was swept up by a mob in Cairo’s Tahrir Square and brutally assaulted.

According to CPJ, at least 18 journalists and their assistants are missing or in government custody in Libya.

Gillis was apparently detained along with James Wright Foley, a correspondent for the Web site GlobalPost, and Manuel Varela de Seijas Brabo, a Spanish photographer. The Libyan government has not acknowledged it is holding them, and diplomats from neutral countries haven’t been able to visit them, as has been customary.

“We simply don’t know when they’ll be released,” said Philip Balboni, the chief executive of GlobalPost. “We’re greatly concerned that they’re there with no knowledge of the diplomatic efforts being made on their behalf. It must be extremely hard for them.”

The Atlantic, USA Today and GlobalPost are coordinating their response through daily conference calls and e-mail lists.

The White House has demanded the release of the reporters. At a briefing this week, press secretary Jay Carney said, “We call . . . for the release of any journalists detained, any human rights activists, anyone detained unlawfully or inappropriately, and in this case, specifically with those journalists in mind, we call on and demand their release.”

Gillis and Foley are U.S. citizens.

Libyan witnesses said they saw government troops detain the three journalists outside Brega, a strategic town in eastern Libya, on April 5, according to Human Rights Watch. Hammerl was originally reported as a member of the group, but it now appears that was erroneous. Hammerl made a phone call to his wife on the evening of April 4; that was the last known contact with him.

The State Department has no diplomatic presence in Libya but is working through Turkish intermediaries. “We are limited in what we can do in Libya right now, except to make public appeals, like I can do right now,” a department spokesman, Mark Toner, said at a press briefing this week. “But beyond that . . . we are limited, unfortunately.”

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